The present invention relates to a vertical-cavity surface emitting laser (VCSEL) diode and its manufacturing method, and more particularly it relates to a surface emitting laser diode where a micro structure for current confinement can be formed precisely and its manufacturing method.
Semiconductor light emitting elements, such as a semiconductor laser and a semiconductor light emitting diode, are commonly used for an optical-communication system, an optical disc system or a bar code reader system, such as CD (Compact Disc) system and DVD (Digital Versatile Disc) system, etc.
Among the semiconductor light emitting elements, a VCSEL diode configures an optical cavity structure using an active layer sandwiched between a pair of mirror stack and emits a laser beam perpendicularly to a surface of semiconductor surface.
In the case of the VCSEL diode, many laser elements can be integrated in two dimensions on the substrate, therefore, it attracts a big attention as a key device in high-speed optical LAN (Local Area Network) and the optical electronics fields, such as optical interconnects.
The following points can be mentioned as the characteristics of the VCSEL diode:
Compared with the conventional edge emitting laser diode, the VCSEL has many advantages such as a low threshold current operation, low power consumption, high slope efficiency, capable of high speed modulation, a low beam divergence for easy connection to fiber optics, requiring no edge cleavage, excellent in mass-production, etc.
The VCSEL requires a current confinement portion for efficiently injecting electric currents into an active region. A method for forming this current confinement portion is to form a high resistance region by means of proton (hydrogen ion) implantation and define a current aperture. Another method proposed is to form a structure comprising a layer to be selectively and laterally oxidized and use a non-oxidized region as a current aperture. These methods are disclosed in Japanese Patent Laid-Open Publication No.H09-266350, Japanese Patent Laid-Open Publication No.2000-332355 and Japanese Patent Laid-Open Publication No.2001-93897.
In the proton implantation method, a small difference of refractive index arises between a current injection region and its circumference by the so-called “thermal lens effect”, with a weak optical confinement. In a VCSEL using proton implantation method, a laser beam with stable lateral transverse mode can be obtained by the weak optical confinement effect even if a diameter size of the confinement portion by proton implantation is about 10 micrometers.
On the other hand, the selectively oxidized VCSEL enables optical confinement as well as current confinement. Then index-guiding optical confinement is obtained because the refractive index is reduced from 3.0 for the original non-oxidized layer to 1.6 for the oxidized layer. For this reason, it is necessary for oxide-confined VCSEL to narrow the diameter of the current confinement portion compared with that for the proton implanted VCSEL, typically to 5 micrometers or less in order to stabilize the lateral transverse mode. That is, in the case of the selective oxidation method, it is required to make the diameter of the oxide-confined aperture minute in order to control the lateral transverse mode.
It is not impossible in fabrication process to form the diameter of the current aperture in 5 micrometers or less. However, since it is difficult to control and reproduce the size and the shape of the diameter of the oxide-confined aperture, it is hard to carry out mass-producing and raising a production yield.
The manufacture method of the oxide-confined VCSEL will be explained briefly, and the control and reproducibility of the size and shape of the diameter of current aperture, which is a problem, will be explained.
First, a laser wafer is made by growing a semiconductor multilayer film reflector, a cladding layer, a semiconductor active layer, a cladding layer, a semiconductor multilayer film reflector, and a contact layer on the semiconductor substrate in this order. The semiconductor multilayer film reflector has a laminated structure of a repetition of an AlxGa1-xAs film/AlyGa1-yAs film, and an AlzGa1-zAs (z>0.95) film as the oxidizee layer (a layer to be oxidized and/or a layer that has been oxidized) with higher aluminum content than the other part of the semiconductor multilayer film reflector.
Next, a mesa structure is formed by etching a semiconductor substrate, and further, a substrate is heated up to 400 degrees centigrade or more in steam atmosphere. Then, the AlGaAs film with higher aluminum content among the semiconductor films, which constitute the semiconductor multilayer film reflector is oxidized selectively from the exposed part of the side of the mesa, and becomes an Al—Ga—O film. The oxidization rate varies remarkably in accordance with the composition of aluminum. For example, if z=0.95−1 in AlzGa1-zAs, aluminum high concentration layer can be oxidized selectively without affecting the cladding layers and other layers hardly.
In the selective oxidation process of the lateral direction, the oxidization of the oxidizee layer proceeds from the side wall of the mesa, and the oxide-confined current aperture is formed at the perimeter part of the mesa, and the part which is not oxidized i.e. the opening is formed in the center of the mesa. The shape and the size of the opening which is not oxidized of the oxidized layer with high aluminum content, can be controlled by adjusting the temperature and time of the heat treatment appropriately.
As explained above, the current aperture for the oxide-confined VCSEL is produced by oxidizing an AlAs layer or an AlGaAs layer with high aluminum content selectively and laterally from the side wall of the mesa structure.
However, an oxidization rate is determined by the substrate temperature, the thickness of oxidizee layer, aluminum composition, the flux of steam, the flux of nitrogen gas, etc., and varies by the conditions of the process much in the selective oxidation process by wet oxidization. For this reason, when carrying out selective oxidation of the AlAs layer or AlGaAs layer with high aluminum content, there is a problem that it is difficult to control the size and shape of a non-oxidized part (opening part) with sufficient reproducibility.
In order to solve this problem, the real time monitoring of the wet oxidation process was tried in order to improve the controllability of oxide-confined aperture size. For example, a method of observing the image of an actual oxide-confined VCSEL with a CCD camera by utilzing the reflectance difference of an AlAs layer and Al2O3 oxide layer due to the refractive-index difference is disclosed by Wright State Univ., IEEE Photon Technol. Lett. 10, p.197(1998).
However, even if this method is used, it is difficult to measure and to control the sizes and the shapes of all oxide-confined apertures of VCSEL diodes over the whole of a semiconductor substrate. It turned out that it is very difficult to control the size and the shape of oxide-confined aperture with high precision, since the oxidization rate changes sensitively with the process conditions and “anisotropy oxidization” which will be explained below arises, especially when forming the current aperture size in 10 micrometers or less.
That is, when using the AlAs layer and AlGaAs layer which contain aluminum in high content as the selective oxidation layer, oxidization along the direction of <100> axes has an oxidization rate higher than the direction of <110> axes in an AlxGa1-xAs (x>0.94) layer. Thus, there is a problem that the shape of a non-oxidizing part changes with oxidization time in wet oxidation process since oxidization rate changes with crystal axis directions. This point will be explained in detail referring to examples.
Moreover, there is a problem that the volume of the oxidizee layer shrinks and the strain is introduced into the upper and the lower layers, when wet oxidization of the AlAs or AlGaAs with high Al content is carried out in the fabrication of oxide-confined VCSEL. Since the volume of oxide layer Alx(Ga)Oy shrinks compared with the Al(Ga)As layer (by about 7% through 13%), compressive stress is applied to the center of the active layer or the mesa structure after oxidization. In order to realize the current confinement effectively, the oxidizee layer as a current blocking layer needs to have a certain amount of thickness. However, the strain becomes larger as this oxidizee layer is made thicker. The strain is concentrated at the tip of the oxide layer. However, since the oxidizee layer is located as close as in only 0.2 micrometers from the active layer, this strain affects the part on which the current injects most in the active layer, and it reduces the life time of the VCSEL diodes.
Especially the tolerance over the heat process after the selective oxidation process falls. Therefore, there may be a problem that the compression stress may be applied to the active layer and the center of the mesa structure owing to the volume shrinking of aluminum high containing layer (the oxidizee layer), and the degradation of reliability, life time, and heat tolerance arise in the conventional oxide-confined VCSEL.
As mentioned above, there is a problem that it is difficult to control the size and the shape of the non-oxidized part used as the current aperture precisely and the laser characteristics, such as a threshold current and a light output, tend to vary in the conventional oxide-confined VCSEL diode.
Furthermore, in the oxide-confined VCSEL diode, there is a problem that since the volume of the oxidizee layer shrinks and strain is introduced into the upper and the lower layers when the wet oxidization of the AlAs or AlGaAs layer with high aluminum content is carried out, the degradation of reliability and life time and heat tolerance of a VCSEL diode arise.